With rare exception, downloading movies without paying for them is illegal. It's called piracy, and if caught doing it the penalties can be severe. With piracy on the rise, movie studios and Internet Service Providers (ISPs) are on the offensive to catch those who pirate movies.
Unless the copyright holder has given permission or the film has no copyright, downloading a movie without paying for it is a crime of copyright infringement and possession of stolen property.
Methods of Capture
It is easy for a person to get caught downloading movies, either by their ISP or by the copyright holders. Most movies are downloaded over P2P services or torrents, so movie studios and ISPs monitor these services and look for users who download illegally. These sites are not anonymous and IP addresses are easily tracked.
Results of Capture
A person caught downloading movies is given a warning for their first offence. This warning usually comes in the form of a letter from their ISP, telling them that they've been monitored downloading copyrighted material and that they could be prosecuted.
Jail Time and Fines
Repeat offenders, or those who download large quantities of movies, may face penalties and punishments for their crimes. In the UK and USA, individuals have been hit with monetary fines in the thousands as well as years of jail time for downloading and sharing movies.
The only movies that can be downloaded legally are those with no copyright. These films are in the public domain. Due to the length of copyrights, most public domain films in America are either very old---from the 30s and 40s---or never had proper copyrights in the first place.